Grinding segment and grinder stone

ABSTRACT

The invention relates to a grinding segment comprising a ceramic part ( 11 ) manufactured of grinding ceramic material and provided with a fastening hole ( 12 ), through which the grinding segment ( 10 ) is attached to a frame ( 9 ) of a grinder stone by means of a fastening bolt. A fastening sleeve ( 14 ) is arranged in the fastening hole, and the fastening bolt is supplied through the sleeve. The fastening sleeve is pressed against the frame of the grinder stone due to the fastening force. A space filled with bonding agent is formed between the ceramic part and the fastening sleeve to join them together. The invention further relates to a grinder stone comprising a cylindrical frame ( 9 ) and grinding segments ( 10 ) according to the invention.

[0001] The invention relates to a grinding segment comprising a ceramicpart manufactured of grinding ceramic material, at least one fasteninghole formed in the ceramic part for fastening the grinding segment bymeans of a fastening element to the outer surface of a frame of agrinder stone used in defibering of wood.

[0002] The invention further relates to a grinder stone for defiberingwood, comprising a cylindrical frame, grinding segments made of grindingceramic material, arranged on the frame's outer circumference andfastened in place by means of a fastening element arranged through atleast one fastening hole formed in the segments, and a shaft, aroundwhich the grinder stone is arranged to be rotated during grinding.

[0003] Wood is typically ground into fibres by means of grindingmachines, where logs are pressed against the surface of a rotatinggrinder stone. Water is simultaneously sprayed to purify and cool thegrinder stone. The stone makes the wood fibre matrix vibrate, whereafterthe wood fibres are released from the logs to provide a pulp suspension.Wood can be ground under both a normal pressure and an overpressure.

[0004] The frame of the grinder stone is usually made of concrete. Agrinding surface consists of separate ceramic grinding segments attachedto the frame. The prior art also teaches arrangements, where concreteframes are replaced with a steel drum. Steel is better able to withstandhigh grinding forces generated during grinding than concrete. Also in asteel drum the grinding segments are attached with suitable boltmechanisms to the outer circumference of the drum to form the grindingsurface of the grinder stone. Different segment fastening mechanisms aredisclosed e.g. in FI 67,240. In this reference, a layer of rubber isdisposed between a grinding segment and the steel drum to balance theloads between the segment and the frame.

[0005] The prior art also teaches different fastening bolt mechanisms,which compensate for varying dimensions resulting from different thermalexpansion coefficients of the ceramic segment and the frame of thegrinder stone. However, such bolts are rather complicated and thereforealso expensive. Furthermore, it is possible to place semi-sphericalwashers between such bolts and the segment in order that the stressesresulting from the fastening will be distributed more evenly in thesegment and the segment will be able to withstand the fastening forces.Failure of a fastening bolt during use results in great damage and aninterruption of production. Another problem with the presentarrangements is that the fastening forces subject the ceramic segment tocompression stress, which can break the segment usually from below thewasher. It is thus necessary to restrict the fastening force, which insome cases can lead to insufficient fastening of the segment. Duringgrinding, such a loose segment can start vibrating and will be damaged.The ceramic materials used in grinding are highly wear-resistant andhard, yet brittle. However, in practice grinding segments cannot bemanufactured from more resistant ceramic materials, since the ceramicmaterial used in grinding of wood must be of a particular type andsufficiently porous to provide a desired grinding effect. Also, qualitycharacteristics set for mechanical pulp require use of the presentgrinding ceramic materials.

[0006] An objective of the present invention is to provide a new andimproved grinding segment and grinder stone for defibering wood.

[0007] A grinding segment according to the invention is characterized inthat a fastening sleeve is arranged in the fastening hole, that a layerof bonding agent is formed between the fastening sleeve and the ceramicpart to join them together, and that the fastening element is arrangedto press the fastening sleeve to the frame of the grinder stone.

[0008] Further, a grinder stone according to the invention ischaracterized in that each grinding segment comprises a fastening sleevein connection with the fastening hole, that the fastening element isarranged to press the fastening sleeve to the frame of the stone, andthat a layer of bonding agent is formed between the fastening sleeve andthe ceramic part to join them together.

[0009] According to a basic idea of the invention, grinding segmentsplaced on the outer surface of the grinder stone comprise a ceramic partmanufactured of a suitable grinding ceramic material and provided withat least one fastening hole for a fastening bolt or some other similarfastening element. A fastening sleeve is arranged in each fasteninghole, so that between the sleeve and the fastening hole there remains aspace, where a layer of suitable bonding agent is formed. The fasteningsleeve and the ceramic part are connected by means of the layer ofbonding agent. The fastening bolts thus press the fastening sleeveagainst the frame of the grinder stone, wherefore the fastening force ofthe segment can be greater than previously, and firmer fastening isprovided. Furthermore, since the fastening element, the fastening sleeveand the frame of the grinder stone are made of the same material,thermal expansion is under control. Also, the layer of bonding agentbetween the sleeve and the ceramic part balances the loads.

[0010] Furthermore, according to a basic idea of a preferred embodimentof the invention, a predetermined proportion of the ceramic part of thegrinding segment, beginning from the bottom of the segment towards thegrinding surface, is impregnated with bonding agent, such as suitableplastic. This avoids the occurrence of breakage and makes the segmentmore capable of withstanding loading. Also, the joint between thefastening sleeve and the ceramic material will be more secure.

[0011] According to a basic idea of another preferred embodiment of theinvention, side surfaces of the grinding segment, which are transverseto the bottom of the segment and possibly also to the grinding surface,are provided with damping layers made of bonding agent. The bottom ofthe segment thus comprises an intermediate layer in the area of theceramic part to balance loads and possible unevenness of the fasteningsurface. When the grinder stone is being used, the side layers in theside surfaces of adjacent grinding segments are positioned tightlyagainst one another, and a point load subjected on an individual segmentis therefore also distributed onto adjacent segments.

[0012] It should be mentioned that in the present application the term‘grinder stone’ refers to the stone frame and the assembly of thegrinding segments and fastening means arranged thereto.

[0013] The invention will be described in more detail in theaccompanying drawings, in which

[0014]FIG. 1 shows schematically a grinding apparatus, where a grindingsegment according to the invention can be used,

[0015]FIG. 2 is a schematic end view of a grinder stone according to theinvention, and FIG. 3 is a schematic side view of a part of the grinderstone according to the invention,

[0016]FIGS. 4 and 5 are schematic sectional views of grinding segmentsaccording to the invention and the fastening thereof,

[0017]FIG. 6 shows schematically placement of segments according to theinvention on the outer circumference of a frame of the grinder stone,

[0018]FIGS. 7 and 8 are schematic sectional side views of embodiments ofthe invention,

[0019]FIG. 9a is a schematic top view of a part of a grinder stoneaccording to the invention, and FIG. 9b shows schematically a grindingsegment according to FIG. 9a viewed from direction A, and

[0020]FIG. 10a is a schematic top view of a grinding segment accordingto the invention, and FIGS. 10b and 10 c show embodiments of thegrinding segment viewed from direction B.

[0021]FIG. 1 shows a grinding apparatus for detaching fibres from logs 1or some other similar wood material by means of a rotating cylindricalgrinder stone 2. The logs 1 are pressed by feeder means, such as feedcylinders 3, from a feed chamber 4 against the outer surface of thegrinder stone 2. Water is simultaneously supplied from nozzles 6 to agrinding chamber 5. The fibre that has been released from the logsaccumulates with the sprayed water in a grinder pit 7 at the bottom ofthe grinding chamber and is conducted therefrom to subsequent processingsteps. The grinding apparatus is considered fully known to a personskilled in the art, wherefore the structure and operation thereof do nothave to be described in more detail herein.

[0022]FIG. 2 shows, in a simplified manner, a grinder stone 2, whichrotates around a shaft 8. The grinder stone comprises a preferably metalcylindrical frame 9, the outer circumference of which is provided withadjacent grinding segments 10 typically made of ceramic, a suitableceramic mixture or some other corresponding material. The segmentsconstitute a grinding surface 30 of the grinder stone that processes thewood. The fastening of the grinding segments to the frame will bedescribed in more detail in FIGS. 4 and 5 below. FIG. 3 is a side viewof a part of the grinder stone.

[0023]FIG. 4 shows a cross-sectional view of the structure of thegrinding segment 10. The segment comprises a ceramic part 11manufactured of ceramic or some other material that is suitable forgrinding. The shape and size of the segment can be selected separatelyin each case. The bottom of the segment is typically curved andcorresponds to the shape of the outer circumference of the stone frame.The outer surface of the segment is also curved, as shown in FIG. 2, forexample. Fastening holes 12 extend through the ceramic part 11 forfastening bolts 13 or some other similar fastening means that arescrewed onto the frame 9 of the grinder stone. Three or more fasteningbolts are preferably used for each segment. According to the basic ideaof the invention, the segment comprises fastening sleeves 14, preferablymade of steel or the like, which coincide with the fastening holes. Afastening bolt goes through a fastening sleeve. For the sake of clarity,the figure shows the fastening sleeve with a rather thick wallstructure. The fastening force provided by the fastening bolt does notdirectly cause compression stress in the ceramic part, but the fasteningbolt tightens the fastening sleeve against the frame of the grinderstone. Since the fastening bolt affects directly the fastening sleeve ofsteel, it is no longer necessary to use such conventional structures assemi-spherical washers for balancing stresses, but a standard lockingwasher 15, such as a spring washer or the like, can be disposed betweenthe sleeve and the bolt. Due to the fastening sleeve, the tighteningforce does not have to be restricted according to the compressionresistance of the ceramic material, but such a high fastening force canbe selected that the segment is guaranteed to stay in place. Anotheradvantage of the arrangement is that the thermal expansion coefficientsof the fastening bolt, the fastening sleeve and the frame of the grinderstone are substantially equal since the components are made of the samematerial, which is typically steel. Therefore thermal expansionresulting from heating of the grinder stone is always under control, andthere is no need for complicated and expensive special bolts tocompensate for thermal expansion.

[0024] As shown further in FIG. 4, a space filled with bonding agent isprovided between the fastening sleeve 14 and the ceramic part 11. Thebonding agent can be for example cast resin, such as vinylester-polyurethane (VEUH), vinyl ester (VE), polyurethane (PU), andepoxy resins. Other possible bonding agents include natural rubber,polystyrene (PS), poly-phenylenesulphide (PPS), polyamide (PA) and othersuitable plastic materials. Moreover, the bonding agent can bereinforced with fibre or it can contain filler. The fibre reinforcementcan consist of such fibre material as a glass fibre mat or a glass fibremesh, staple glass fibre, milled glass fibre, kevlar, carbon fibre orsome other suitable fibre material. The filler can be aluminium oxide,silica sand, kaolin, talc or some other suitable filler. In some cases,different cements and metallic soldering materials can also be used asbonding agent. The fastening force is transmitted from the fasteningsleeve to the ceramic part via the aforementioned layer 16 of bondingagent. The layer has a thickness of 0.1 to 3 mm, preferably 0.3 to 1 mm.The layer of bonding agent is also able to compensate for deviationsbetween the ceramic part 11 and the fastening sleeve 14 resulting fromdifferent thermal expansion coefficients of the materials. Furthermore,the bottom of the ceramic part is preferably provided with a layer ofcorresponding bonding agent, which forms a damping intermediate layer 20between the segment and the frame. The intermediate layer dampspercussive loads possibly directed at the segment during the grinding ofwood, thus preventing segment breakage. It also prevents vibration inthe segment. Furthermore, the intermediate layer 20 balances any slightunevenness that possibly occurs between the joint surfaces of thesegment and the frame of the grinder stone, thus improving thefastening. Due to the intermediate layer, the segment bottom and thefastening surface formed in the frame of the grinder stone do not haveto be perfectly finished during the manufacture, which reduces the costsof manufacture. The intermediate layer has a thickness of preferably 1to 3 mm. On the other hand, the intermediate layer is not necessary.

[0025] The segment shown in FIG. 5 mainly corresponds to the segmentshown in FIG. 4. However, in FIG. 5 a proportion of the bottom section17 of the segment is impregnated with plastic bonding agent beginningfrom the segment's bottom towards a grinding surface 30 of the segment.The impregnated bottom section 17 prevents the formation of ruptures inthe segments and improves the joint between the ceramic part and thefastening sleeve. The impregnation is preferably carried out from thelower surface of the segment approximately to the level of the upper endof the fastening sleeves, but it can also be performed closer to theupper surface of the segment. For the sake of illustration, a brokenline 18 in the figure shows different possible locations of theinterface between the impregnated bottom section 17 and the surfacesection 19. Between the grinding surface 30 and the impregnated part 17of the segment there is a nonimpregnated section with good grindingproperties. The grinding section of the grinder stone can be sharpenedas the need arises, so that when the segment is finally replaced as aresult of normal wearing, the grinding section has been used all the wayto the impregnated section. Furthermore, the segment side surfaces arepreferably provided with side layers 21 formed of bonding agent, whichact as damping parts between adjacent segments. The side layers 21 havea thickness of preferably 1 to 3 mm and they extend preferablysubstantially to the level of the impregnated bottom section 17 of thesegment. The side layers are preferably arranged in all the sides of thesegment that are transverse to the grinding surface. If required, someor all of the side layers can be eliminated. Furthermore, the fasteningsleeves 14 of the segment according to FIG. 5 differ from thecylindrical sleeves shown in FIG. 4. In FIG. 5, the outer surfaces ofthe fastening sleeves comprise one or more protrusions 28. The sleeve'souter surface is preferably wavelike, and the wave's cross-section issimilar to a sine wave. The wave shape can also be similar to a squarewave, a saw-tooth wave, etc. The protrusion can also resemble a thread.Correspondingly, the circumference of the fastening hole formed in theceramic part can be provided with one or more protrusions in the sectioncorresponding to the level of the fastening sleeve, the protrusionspreferably corresponding to those in the sleeve. The shapes of theprotrusions in the ceramic part and in the fastening sleeve can alsodiffer from one another, if necessary, and protrusions can only beprovided in one element and not in the other. The purpose of theprotrusions is to improve the joint between the ceramic part and thefastening sleeve, and the shape of the protrusions can be used to adjustthe behaviour of the segments under stress.

[0026] The intermediate layer 20 and the side layers 21 shown in FIG. 5can also be components made of suitable bonding agent and separate fromthe ceramic part 11. Furthermore, the bonding agent used in theintermediate layer 20, the side layers 21 and the space between thefastening sleeve 14 and the ceramic part 11 can be reinforced with glassfibre or the like.

[0027]FIG. 6 shows a preferred arrangement for fastening segments. Theouter circumference of the frame 9 of the grinder stone is provided withplanar sections 22 a to 22 c, parallel to the shaft of the grinderstone, for the segments. The planar sections are also provided withfastening holes 23, so that the segments 10 can be attached to the frameof the grinder stone by means of fastening bolts 13. As shown in thefigure, each segment is provided with three fastening bolts to ensurestable fastening that prevents swaying. Viewed in the direction of theradius of the grinder stone, the segments 9 shown in the figure comprisea base 24, two sections 25 a and 25 b that are perpendicular to thebase, and a triangular apex 26 extending from the sections. Such sectorscan be arranged in the planar sections of the frame in alternatingdirections, such that the bases 24 are parallel to the frame shaft or tothe edge of the planar section, and the apices of adjacent segments thuspoint in opposite directions. In such a manner the segments can bepositioned tightly against one another to constitute a grinding surface.The segments of the entire grinder stone can thus be of the same shape,excluding end segments 27 to be placed at the end of the grinder stone.When damaged or worn-out segments are being replaced, it is notnecessary to remove the grinder stone from the grinding apparatus, butsegments can be detached and attached while the stone is in place, e.g.via an opening provided in the grinding apparatus for sharpening thegrinder stone.

[0028]FIG. 7 shows an arrangement, where a transverse support element 40is arranged between the upper end of the fastening sleeve 14 and thefastening bolt 13 to secure the fastening of the segment. The segment isprovided with an opening, transverse to the fastening hole 12, forreceiving the support element 40. Bonding agent 16 is applied betweenthe support element and the transverse opening. The support element ispreferably rectangular, but the shape can be selected suitably in eachcase.

[0029]FIG. 8 shows a conical fastening sleeve 14, which is arranged withthe greater sleeve diameter on the side of the segment's upper surface.The conical fastening sleeve ensures firm fastening of the segment. Thefastening sleeve 14 can also be a separate piece, which is precoatedwith a required layer 16 of bonding agent and thereafter arranged in afastening hole 12 in the ceramic part 11. When the fastening element 13is being tightened, the layer 16 of bonding agent in the fasteningsleeve is pressed against the ceramic part 11 to join the part to thefastening sleeve. On the other hand, the layer of bonding agent betweenthe fastening sleeve and the ceramic part can be a preformed separatepiece.

[0030] The grinder stone 2 shown in FIG. 9a comprises rectangulargrinding segments 10, which are attached to the stone's outer surfacewith two fastening bolts or a corresponding fastening element. As shownin FIG. 9b, the grinding segments 10 have a curved bottom surface, sothat the segment rests against the curved outer surface of the frame 9of the grinder stone.

[0031]FIGS. 10a to 10 c show grinding segments 10, where the bottomscomprise one or more protrusions 50 or grooves 51. The outer surface ofthe frame 9 of the grinder stone is also provided with sections that fitin the aforementioned protrusions or grooves, so that shear forcesproduced during the use of the grinder stone can be transmitted from thegrinding segment to the stone frame. Such fastening of the grindingsegment is firm, and only one tightening bolt per segment may besufficient. In the arrangements shown in FIGS. 9a to 10 c, eachfastening hole is provided with a fastening sleeve according to theinvention.

[0032] The drawings and the related description are only intended toillustrate the inventive idea. The details of the invention can varywithin the scope of the claims. It is thus clear that even if thesegments advantageously have a similar shape as shown in FIG. 6, thefastening arrangement according to the invention can also be applied inconnection with segments of other shapes. The bonding agent to be usedbetween the fastening sleeve and the ceramic part, the bonding agentthat impregnates the segment, the bonding agent in the bottom of thesegment, and the bonding agents in the side layers of the segment can bedifferent in some cases, although with respect to the manufacturingtechnique it is advantageous to mould all the aforementioned layers ofbonding agent simultaneously. Furthermore, the intermediate layer can bemade to extend beyond the fastening sleeve at the bottom of the segment,so that when the segment is tightened in place, the intermediate layeris compressed to some extent so as to produce a predetermined initialtension, which prevents occurrence of vibrations. Also, even though theinvention is described in the figures in connection with a grinder stoneof a steel frame, the invention can also be applied in grinder stoneswith a frame of concrete.

1. A grinding segment comprising a ceramic part (11) manufactured ofgrinding ceramic material, at least one fastening hole (12) formed inthe ceramic part for fastening the grinding segment by means of afastening element (13) to the outer surface of a frame (9) of a grinderstone used in defibering of wood, characterized in that a fasteningsleeve (14) is arranged in the fastening hole (12), that a layer (16) ofbonding agent is formed between the fastening sleeve and the ceramicpart to join them together, and that the fastening element is arrangedto press the fastening sleeve to the frame of the grinder stone.
 2. Agrinding segment according to claim 1, characterized in that the outersurface of the fastening sleeve (14) is provided with protrusions (28).3. A grinding segment according to claim 1 or 2, characterized in thatthe inner surface of the fastening hole (12) formed in the ceramic part(11) is provided with protrusions (28) over a section corresponding tothe fastening sleeve (14).
 4. A grinding segment according to any one ofthe preceding claims, characterized in that the bottom of the grindingsegment in the area of the ceramic part (11) is provided with anintermediate layer (20) of bonding agent arranged to remain between theframe (9) of the grinder stone and the ceramic part (11) of the grindingsegment when the segment is installed in place.
 5. A grinding segmentaccording to any one of the preceding claims, characterized in that atleast some of the sides of the grinding segment, transverse to agrinding surface (30), are provided with side layers (21) of bondingagent, which are arranged to be disposed towards adjacent grindingsegments when the segment is in place in the grinder stone.
 6. Agrinding segment according to any one of the preceding claims,characterized in that a predetermined proportion of the ceramic part ofthe grinding segment is impregnated with bonding agent beginning fromthe bottom of the segment towards the grinding surface (30) thereof. 7.A grinding segment according to claim 6, characterized in that aproportion of the grinding segment that corresponds substantially to thefastening sleeves is impregnated with bonding agent beginning from thebottom of the segment towards the grinding surface (30).
 8. A grindingsegment according to any one of the preceding claims, characterized inthat the bonding agent is reinforced with fibre.
 9. A grinder stone fordefibering wood, comprising a cylindrical frame (9), grinding segments(10) made of grinding ceramic material, arranged on the frame's outercircumference and fastened in place by means of a fastening element (13)arranged through at least one fastening hole (12) formed in thesegments, and a shaft (8), around which the grinder stone is arranged tobe rotated during grinding, characterized in that each grinding segmentcomprises a fastening sleeve (14) in connection with the fastening hole,that the fastening element is arranged to press the fastening sleeve tothe frame (9) of the stone, and that a layer of bonding agent (16) isformed between the fastening sleeve and the ceramic part to join themtogether.
 10. A grinder stone according to claim 9, characterized inthat the outer circumference of the frame (9) of the grinder stone isprovided with planar sections (22 a-22 c), parallel to the shaft (8) ofthe stone, for fastening the grinding segments (10).